The present invention relates to the field of flexible wiring boards, particularly to the technique of laminating single-layer flexible wiring boards to form a multilayer flexible wiring board of laminated structure.
Recently, multilayer flexible wiring boards are widely used in the field of electronic apparatus.
A method for obtaining a multilayer flexible wiring board involves laminating a plurality of single-layer wiring boards.
In order to laminate single-layer wiring boards to form a multilayer flexible wiring board, we began by preparing first and second flexible wiring boards 110, 120 as described below on trial.
Referring to FIG. 8(a), first flexible wiring board 110 has a first base film 111, on which a first conductive film 112 is formed by patterning a copper foil.
A gold coating 113 is formed on the surface of first conductive film 112 so that gold coating 113 and first conductive film 112 form a first wiring film 119. A plurality of such first wiring films 119 are formed.
The reference 114 in FIG. 8(a) represents an opening region including an opening or space located between first wiring films 119, which are insulated from each other by said opening region 114. First base film 111 is exposed at the bottom of opening region 114.
The reference 120 in FIG. 8(b) represents a second flexible wiring board to be laminated to said first flexible wiring board 110.
This second flexible wiring board 120 has a second base film 121, on which a second conductive film 122 is formed by patterning a copper foil. A resin film 123 including a thermoplastic resin having insulating properties is formed on the surface of second conductive film 122, and a metal projection 125 is mounted on second conductive film 122 with the top projecting from the surface of resin film 123.
A low-melting metal coating 126 including solder is formed on the surface of metal projection 125 so that low-melting metal coating 126 and metal projection 125 form a bump 124.
For laminating said first and second flexible wiring boards 110, 120, these flexible wiring boards are first arranged in such a manner that first conductive film 112 of first flexible wiring board 110 and bump 124 on second flexible wiring board 120 face each other (FIG. 9(a)).
Then, gold coating 113 on first conductive film 112 and low-melting metal coating 126 on bump 124 are contacted with each other (FIG. 9(b)) and heated under pressure until low-melting metal coating 126 melts. Then, low-melting metal (solder in this example) is cooled to solidify to connect first conductive film 112 and bump 24.
Thermoplastic resins soften to develop adhesiveness upon eating. Resin film 123 softens as low-melting metal coating 126 melts, and then it is cooled to bond two flexible wiring boards 110, 120 via resin film 123. As a result, first and second conductive films 112, 122 are electrically connected via bump 124 into a multilayer flexible wiring board 130 (FIG. 9(c)).
However, there is a demand for reducing the space between wiring films of flexible wiring boards to be heih-density.
In case of multilayer flexible wiring board 130 having the structure described above, low-melting metal coating 126 melts under pressure to sometimes fly about.
If the space between first wiring films 119 or between second conductive films 122 is narrow, the flying low-melting metal may form a bridge 127 to invite short circuit. In FIG. 9(c), the resulting bridge 127 causes short circuit between first wiring films 119.
An object of the present invention is to provide a technique for laminating flexible wiring boards at high yield to overcome the disadvantages of the prior art described above.
In order to attain the above object, the present invention provides a multilayer flexible wiring board comprising laminated first and second flexible wiring boards, said first flexible wiring board comprising a first base film, a first wiring film having a first conductive film applied on said first base film and patterned, a contact pad including the part of said first wiring film exposed at the surface, and a wall member provided around said contact pad and rising above the top of said contact pad, and said second flexible wiring board comprising a second base film, a second wiring film having a second conductive film applied on said second base film and patterned, and a plurality of bumps connected to said second wiring film and exposed at the top, wherein a low-melting metal coating is formed on either of the surface of the part of said first conductive film forming said contact pad or the surface of at least the top of said bump, and said low-melting metal coating melts under heat and then solidifies to connect said contact pad and said bump.
In the multilayer flexible wiring board of the present invention, said low-melting metal may be an alloy based on lead and tin.
In the multilayer flexible wiring board of the present invention, said low-melting metal may be an alloy based on tin and gold but not including lead.
In the multilayer flexible wiring board of the present invention, a highly wetting metal coating including a metal or alloy being wetter than copper with said low-melting metal coating, may be formed on the surface of the part of said metal film forming said contact pad or the surface of at least the top of said bump on which said low-melting metal coating is not formed.
In the multilayer flexible wiring board of the present invention, said highly wetting metal coating may be any of a gold coating based on gold, a platinum coating based on platinum, a silver coating based on silver, a nickel coating based on nickel, a copper-nickel alloy coating or a solder coating.
In the multilayer flexible wiring board of the present invention, the height of said wall member above the top of said contact pad may be 5 xcexcm or more.
In the multilayer flexible wiring board of the present invention, a first resin film is provided on said first wiring film and an opening is provided in said first resin film at the top of said contact pad so that said wall member includes said first resin film forming the inner circumferential face of said opening.
In the multilayer flexible wiring board of the present invention, said first resin film has the property of developing adhesiveness upon heating so that said first flexible wiring board and said second flexible wiring board are bonded together via said first resin film.
Said opening of the multilayer flexible wiring board of the present invention has such a volume that said low-melting metal coating does not overflow outside said opening when said low-melting metal coating is molten while said bump is in contact with said contact pad.
Said opening of the multilayer flexible wiring board of the present invention has such shape and area that said bump can come into contact with said contact pad without touching said wall member.
The height of the projecting part of said bump of the multilayer flexible wiring board of the present invention is larger than the height of said wall member from the top of said contact pad.
Said second flexible wiring board of the multilayer flexible wiring board of the present invention has at least a second resin film provided on at least said second wiring film and the top of said bump projects from said second resin film.
Said second resin film of the multilayer flexible wiring board of the present invention has the property of developing adhesiveness upon heating so that said first flexible wiring board and said second flexible wiring board are bonded together via said second resin film.
The multilayer flexible wiring board of the present invention has the structure described above, comprising first and second wiring films wherein a low-melting metal coating is formed on at least one of the surface of the contact pad at the top of the first wiring film or the surface of the bump. A wall member rising above the top of the contact pad is provided around the first wiring film exposed at the top of the contact pad.
When the first and second flexible wiring boards are superposed and heated while the top of the bump is in contact with the top of the contact pad, the low-melting metal coating melts but the melt of the low-melting metal coating is stopped by the wall member from flowing out. Therefore, no short circuit occurs between adjacent wiring films.
The height of the wall member from the top of the contact pad is preferably 5 xcexcm (5xc3x9710xe2x88x926 m) or more.
Considering that wiring films are generally made from copper, the multilayer flexible wiring board of the present invention has a highly wetting metal coating being wetter than copper with the low-melting metal coating on the surface of the contact pad or the surface the bump on which the low-melting metal coating is not formed. Specifically, such a highly wetting metal coating may be a metal coating based on gold, a metal coating based on silver, a metal coating based on nickel, a solder coating, a copper-nickel alloy coating, etc.
When the first and second flexible wiring boards are heated to allow the low-melting metal coating to melt, the melt spreads over the surface of the highly wetting metal coating and then solidifies to firmly connect the bump and wiring films.
The wall member of the present invention may be provided at the top of the contact pad or a location apart from the contact pad such as at the top of the base film. The wall member should desirably have a structure surrounding the contact pad.
The first and second flexible wiring board, and the multilayer flexible wiring board of the present invention have property of flexibility.